Is it ok to use 10% benzoyl peroxide when having acne after hair transplant and for how long?

Ask your doctor.  You should not be using benzoyl peroxide on your scalp.  You can use it where you did not receive the transplant.  Follow up with your doctor.  Donor hair is gone for good.  It does not "come back".  You paid a lot of money for a surgery to move it to another location.  Donor hairs do not grow back.  If you are referring to shock hair loss around the donor area, it may take up to a year.

Peroxide can damage the growth of the hair grafts. The use of peroxide on the acne depends On Where you are getting the acne. To be sure it is safe to see your dermatologist for the acne  Treatment.